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Cookie authentication is the standard authentication method included with WordPress. When you log in to your dashboard, this sets up the cookies correctly for you, so plugin and theme developers need only to have a logged-in user.

However, the REST API includes a technique called nonces to avoid CSRF issues. This prevents other sites from forcing you to perform actions without explicitly intending to do so. This requires slightly special handling for the API.

For developers using the built-in Javascript API, this is handled automatically for you. This is the recommended way to use the API for plugins and themes. Custom data models can extend wp.api.models.Base to ensure this is sent correctly for any custom requests.

For developers making manual Ajax requests, the nonce will need to be passed with each request. The API uses nonces with the action set to wp_rest. These can then be passed to the API via the _wpnonce data parameter (either POST data or in the query for GET requests), or via the X-WP-Nonce header. If no nonce is provided the API will set the current user to 0, turning the request into an unauthenticated request, even if you’re logged into WordPress.

Note: Until recently, most software had spotty support for DELETE requests. For instance, PHP doesn’t transform the request body of a DELETE request into a super global. As such, supplying the nonce as a header is the most reliable approach.

It is important to keep in mind that this authentication method relies on WordPress cookies. As a result this method is only applicable when the REST API is used inside of WordPress and the current user is logged in. In addition, the current user must have the appropriate capability to perform the action being performed.

As an example, this is how the built-in Javascript client creates the nonce:

While cookie authentication is the only authentication mechanism available natively within WordPress, plugins may be added to support alternative modes of authentication that will work from remote applications. Some example plugins are OAuth 1.0a Server, Application Passwords, and JSON Web Tokens.